ALUMINA FLOC Composition of Floc Formed at pH Values below 5.5 HARRY B. WEISER, W. 0. MILLIGAN, W. R. PURCELL
AND
The Rice Institute, Houston, Texas
I
N T H E previous paper of this series (IO), it was pointed out that the alumina floc thrown down from aluminum sulfate solution between pH values of 5.5 and 8.5 consists of hydrous agglomerates of minute crystals of yALO,.H20. Although there was no indication of the formation of a basic aluminum sulfate under the usual conditions of floc formation, it does not follow that definite basic salts cannot be formed under other conditions. This paper is concerned with the composition of precipitates thrown down from the aluminum salt solutions below a pH of 5.5. Hildebrand (8-compare Davis and Farnham, 2 ) found a slight break a t a pH of about 4 in the potentiometric titration of aluminum sulfate with sodium hydroxide. Miller ( 5 ) , Williamson (II), and Hopkins (4) observed that the ratio of Also3 to SOs has an almost constant value of 5 to 3 in thoroughly washed gels thrown down from aluminum sulfate solutions in the p H range 4 t o 5.5. The break in the titra-. tion curve and the constancy of the composition of the precipitate formed below pH 5.5 suggest that it may be a basic salt. On the other hand, this view is open to question because of (a) the ease with which the sulfate is displaced by washing the precipitate with solutions of negative ions of equal or greater valence ( I ) , and (b) the possibility that adsorption of aluminum and hydrogen ions attains the saturation point a t pH values of 5.0-5.5 and lower (6). Since doubt exists as to the constitution of the alumina floc precipitated below a pH of 5.0-5.5, a systematic x-ray diffraction study was made of the gels formed from aluminum sulfate, nitrate, and chloride.
Potentiometric Titrations Varying amounts of 1.8 M sodium hydroxide were added dropwise to a rapidly stirred solution of 250 cc. of 0.04 M aluminum sulfate. The pH of the suspension was measured after the addition of each cubic centimeter of base, using a Beckman glass electrode apparatus. The instrument was calibrated against a freshly prepared buffer of potassium acid phthalate obtained from the National Bureau of Standards. The potentiometric titration curve is given in Figure 1. I n a similar manner 1000 cc. of 0.02 M aluminum chloride and 1000 cc. of 0.02 M aluminum nitrate were titrated with 1.8 M sodium hydroxide. Each curve shows a break a t a pH of about 4 (Figure l), in confirmation of Hildebrand’s result for the sulfate.
Gels from Aluminum Sulfate at pH Values below 5.5 PREPARATION OF SAMPLES.Samples of gels corresponding to various points along the aluminum sulfate titration curve were prepared in amounts large enough for x-ray and chemical analysis. Separate 250-cc. portions of 0.04 M aluminum sulfate solutions were treated with varying amounts of 1.8 M sodium hydroxide, and the pH of each mixture was determined.
FIGURE1. TITRATION CURVESFOR ALUMINUMSULFATE, AND CHLORIDE NITRATE, 669
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INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE2. X-RAYDIFFEACTION PATTERNS FOR GELS FROM ALUMINUM SULFATE PRECIPITATED
AGIXG OF SAMPLES AT 100" C. Preliminary x-ray diffraction studies showed that only two or three broad bands, not in the position of the y-A1203.H20bands, were obtained for the washed and air-dried samples prepared as described above. To obtain a larger primary particle size, the suspensions of gel in the mother liquor were boiled for 24 hours in a flask fitted with a reflux condenser. The resulting aged samples were washed on a Buchner funnel until the filtrate was sulfate-free and were dried at 110' C.
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X-RAYANALYSIS. X-radiograms were obtained for the samples prepared, aged, and dried as described above, using filtered Cu K, x-radiation and no-screen films. The exposure time was 30 minutes. Some of the negatives are reproduced in Figure 2. Above a pH of 5.5 the standard pattern of y-Alr03.Hzo is obtained, in accordance with the results previously reported (IO). On the other hand, below a pH of 5.5 the pattern of a new crystalline phase appears. This pattern does not correspond to any of the recognized forms of hydrated or anhydrous alumina (7'). ATTEMPTS TO GROW LARGECRYSTALS.A portion of a suspension precipitated from aluminum sulfate a t a pH of 4.5 was sealed in a Pyrex tube and heated for 24 hours a t 180' C. in a Carius furnace. The x-ray diffraction pattern of the product indicated only a slight increase in primary particle size. A quantity of a washed and dried sample originally precipitated a t plI 4.5 was suspended in water and heated in a steel bomb to 500" C. a t a pressure of 9000 pounds per square inch for 7 hours. The x-ray diffraction pattern of the resulting product likewise showed little increase in primary particle size. SAMPLES FROM ALUM SOLUTIOPI'S. X-ray diffraction patterns were obtained for samples prepared by the addition of sodium hydroxide to solutions of potassium, sodium, and ammvriium alums. Below a pH of about 5.5 the patterns of the samples aged by boiling were identical with those from pure aluminum sulfate solution. The exact limits for the formation of the new crystalline phase from the alum solutions were not determined. CHEMICAL ANALYSIS. The size of the primaiy particles in the products aged by boiling was estimated to be just within the colloidal range, and it was assumed that the adsorption of aluminum and sulfate ions was small enough to make a chemical analysis worth while. The contents of AlzOa and SO3 were estimated by standard analytical procedures; water was determined by dlfference. Samples originally precipitated a t a pH of 4.5, aged at the boiling point of the mother liquor for 24 hours, thoroughly washed, and dried a t 110" C., correspond closely to the formula A&O3.SO3.zH20,The ratio of -&O3 to SO8 varies slightly for different samples because of unavoidable variations in the aging and washing procedure. A typical analytical result is 49.4 per cent & 0 3 , 36.2 SOS, 14.4 HzO, corresponding to the formula A120a.0.93 S03.1.f35H,0. It is probable that the ratio of combined AlpOQ:S03is 1, and that the observed variation from this ratio with different samples results from varying amounts of adsorption on crystals of different size. DEHYDR-4TIOX ISOBAR. -4 dehydration isobar was obtained for a sample prepared as indicated in the preceding paragraph, using methods already described (8, 9). The isobar is given in Figure 3. No loss of SO3 mas observed a t 600' C. A final point taken a t 700' C., but not included in the figure, shows a loss in weight corresponding closely to the sum of the H2O and SO3 contents as determined by chemical analysis. The results indicate that 1.5 moles of water are chemically combined with 1 mole of AlzOa.S03. A fkal decision as to the exact amount of combined water must await the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
preparation of larger crystals, which are expected to give a sharper break in the dehydration isobar.
Gels from Aluminum Nitrate and Chloride at pH Values below 5.5 Since the titration curves of aluminum nitrate and chloride are similar to that of aluminum sulfate, experiments were carried out to determine whether the precipitates thrown down from chloride and nitrate solutions a t low p H values consisted of basic aluminum salts. The samples were precipitatedat apHof4.0, andwerewashedanddriedinthesame way as the precipitates from the sulfate. The x-ray diffraction pattern (Figure 4) of the unaged but carefully washed and air-dried gels from chloride and nitrate corresponded to the pattern of y-A1-$&..HzO. It has already been reported (7, IO) that the y-A120a.H20precipitated from the chloride or nitrate gives a much sharper x-ray diffraction pattern than that precipitated from the sulfate. Since the washed and air-dried gels precipitated from the nitrate and chloride a t pH 4 were identified as being y-AlzOa.HzO,further studies were unnecessary. It is recognized that these gels contain considerable amounts of adsorbed nitrate and chloride, respectively. If the gels of y-A1203.Hz0 precipitated from aluminum chloride or nitrate contain any basic salt, it is either amorphous to x-rays or, if crystalhe, is present in too small quantities to be detected by x-ray analysis.
of combined water must await the preparation of macroscopic crystals which are expected t o adsorb less water and therefore to give a sharper break in the dehydration curve. I n the light of existing information the basic salt may be represented by the formula &0a.S03.1.5Hp0. The conclusion that the basic salt has the formula Al2O3.SOs.1.5Hz0may appear t o be inconsistent with the analytical results of Miller, Williamson, and Hopkins who assigned the
Standard cutrate
Nitrate of pH 4
Standard chloride
Chloride of p H 4
Discussion The x-radiograms of the freshly formed floc from aluminum sulfate solution precipitated below a pH of 5.5 consist of two or three bands that are not in the same position as the diffraction bands obtained from minute crystals of yA120S.H20. Aging of the gel from sulfate solution in the boiling supernata.nt solution increases the primary particle size until, after 24 hours, the sample gives a relatively sharp pattern which is distinct from that obtained from any recognized form of hydrated or anhydrous alumina. This new
TEMPERATURE, DEGREES c. FIQURFJ 3. DEHYDRATION ISOBAR crystalline phase must be either a new crystalline form of alumina or a definite basic sulfate. The results obtained from the dehydration isobar indicate that the new crystalline phase contains both chemically combined sulfate and water (or the elements of water). Chemical analysis of the relatively large crystals of the aged precipitate show that the mole ratio of AlsOa to SO3 is approximately 1. It is believed that a small variation from this ratio, such as the analysis disclosed, is due to contamination by adsorption. The exact determination of the amount
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FIGURE 4. X-RAYDIFFRACTION PATTERNS FOR GELSPRECIPITATED FROM ALUMINUM NITRATE AND CHLORIDE formula 5Al,0~.3S03to the unaged gel. However, the fresh gel precipitated below a pH of 5.0-5.5 is formed in the presence of relatively high concentrations of aluminum ions, which would be adsorbed strongly by the highly dispersed gel. The constant ratio of Ah08 to so8 in the fresh gels prepared by M i e r , Williamson, and Hopkins would follow if the adsorption of aluminum ion were a t the saturation value a t pH 5.0-5.5 and lower. The formula 5Al2o3.3SO8for the fresh gel cannot be correct for two reasons. I n the first place, the fresh, highly dispersed, and highly adsorptive gel is certain to be contaminated greatly by adsorption, and the analytical results give only the ratio A l 2 0 ~ so8 : without distinguishing between adsorbed and combined ions. Secondly, the x-ray diffraction bands for the fresh, highly dispersed gel are in the same position as the sharper lines in the aged gel. As already pointed out, the patterns of both the fresh and aged gels are different from that of y-&O3.H20. The x-ray results prove that a corresponding basic nitrate or chloride is not obtained under conditions that yield a definite basic sulfate. Since a similar break in the titration curve is obtained for the three aluminum salts, it follows that such a break is not conclusive evidence of the formation of a basic salt. In accordance with the views of Davis and Farnham ( 2 ) , such breaks in the titration curves of aluminum salts are attributed to free acid.
Summary Briefly, the results of this investigation are as follows: 1. An earlier conclusion i s confirmed that the alumina floc in water purification practice is ~-AIzO~.HZO, free of basic aluminum sulfate.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2. Potentiometric titration curves for aluminum sulfate, nitrate, and chloride solutions show a break a t a pH of about 4. This suggested the possibility that definite basic salts of aluminum are formed, but it was found that such breaks in titration curves do not furnish conclusive evidence of basic salt formation. 3. X-ray diffraction patterns of flocs freshly formed from aluminum sulfate solution at pH values below 5.5 show two or three broad bands which do not correspond to lines in the pattern for y-A1&.H20. 4. Aging of the preci itate referred to in paragraph 3 in the mother liquor at the boigng point increases the primary particle size; after 24 hours the precipitate gives a relatively sharp x-ray diffraction pattern. 5. The x-ray diffraction pattern of the aged precipitate from sulfate is distinct from any recognized form of hydrated or anhydrous alumina. Therefore, it must be either a new form of alumina or a definite basic sulfate. 6 . The results.obtained from chemical analysis and a dehydration isobar of the aged precipitate from sulfate show that the new crystalline phase is a definite basic sulfate which may be represented by the formula A12O~.S0~.1.5H20. 7. There is no indication that more than one definite basic aluminum sulfate exists. 8. Gels prepared from aluminum nitrate and aluminum chloride solutions at a pH of about 4.0 have been found to be y-
A1203.H10. There is no indication that basic aluminum nitrate or chloride can be prepared under conditions that give a definite basic sulfate. Accordingly, breaks in potentiometric titration curves do not furnish conclusive evidence of the formation of basic aluminum salts. The observed breaks result from the presence of free acid in the original aluminum salt.
Literature Cited (1) Charriou, Compt. rend., 176, 679, 1890 (1923). (2) Davis and Farnham, J. Phys. Chem., 36, 1057 (1932). (3) Hildebrand, J. Am. Chem. Soc., 35, 847 (1913). (4) Hopkins, J . Am. Water Works Assoc., 12, 405 (1924). (5) Miller, U.S. Pub. Health Repts.,38, 1995 (1923). (6) Weiser, "Inorganic Colloid Chemistry", Vol. 11, p. 401, New York, John Wiley & Sons, 1935. (7) Weiser and Milligan, Chem. Rev.,25, 1 (1939). (8) Weiser and Milligan, J. P h w . Chem., 38, 513 (1934). (9) Weiser, Milligan, and Ekholm, J. A m . Chem. Soc., 58, 1261 (1936). (10) Weiser, Milligan, and Purcell, IXD. ENC. CHEM..32. 1487
(1940). (11)
Williamson, J . Phys. Chem., 27, 284
(1923).
PRXOSWNTELI before the Division of Colloid Chemistry a t the 100th Meeting of the American Chemical Society, Detroit, hlich.
COAL HYDROGENATION Natural Bituminous Coal; Artificial Cellulose Coal and Its Bitumen and Residual Portion E. BERL, H. BIEBESHEIMER, AND W. KOERBER Carnegie Institute of Technology, Pittsburgh, Penna.
H
YDROGEKATIOK of natural bituminous coals and lignites is carried out on a large industrial scale. Many scientific articles concerning hydrogenation of natural solid fuels have been published, including U. S. Bureau of Mines publications b y Xtorch and collaborators. This paper describes experiments on the hydrogenation of natural bituminous coals and wood charcoal, and the subsequent hydrogenation of bitumen and residual coal made from these natural bituminous coals by extraction with tetrahydronaphthalene under pressure a t 250" C. (3, 4). Before hydrogenation the adhering tetrahydronaphthalene was carefully removed from the bitumen and the residual coals. Iron oxide, iron, TABLE I. COMPOSITION OB MATERL4LS % C 84.2
85.9 76.27 84.4
% H 5.95 5.90 6.30 3.7
% O + S + N 9.85 8.30
17.43
and iodine were used as catalysts. Tables I and I1 show the composition of the materials, the properties of the hydrocarbons, and the results obtained. Thus from the separate hydrogenation of the components of bituminous coals (bitumen and residual coal), considerably larger amounts of liquid fuels are obtainable (about 22.5 per cent or more) than from hydrogenation of the original coals. With different original material, different conditions of hydrogenation, and different catalysts, results which are different, but not fundamentally so, may be obtained. The reason for the higher yield in hydrogenating residual coal and bitumen separately may be the creation of new and increased surfaces accessible to the attack of hydrogenating hydrogen by the extraction of bitumen. It is remarkable that wood charcoal, which has not been heated above 500" C., yields practically pure methane (1) upon hydrogenation. Table I11 shows the calculated data of the reaction:
11 9
C TABLE 11. RESULTS OF HYDROGENATION Original Bituminous Coal, A
a
6
Grams hydrogenated 100 Hydrogen prespre, atm. 103 Temperature, C. 445/50 Time, minutes 120 Liquid hydrogenation 49.6 products, grams Liquid hydrogenation,pfod49.6 ucts from 100 g. original material, grams C = 100. B B + C = 60.79 = 122.570 of 49.6
-+
Residual Coal, B 83 103 480 120 44.38
53.46
Bitumen, C 17a 103 440/50 120
16.41b 96.6
+ 2Ha
--L
CHa
+ 18.11kg.-cal.
Figure 1 shows the results obtained by the hydrogenation of activated (peat) carbon and beechwood charcoal at 500" C. under different hydrogen pressures. The high theoretical yield (over 90 per cent) of methane was not attained, but up to 80 per cent yield was observed. The carbon contents of residual coal and wood charcoal are nearly the same (Table I). The hydrogenation products are fundamentally different. Residual coal gives 53.46 per cent of liquid hydrocarbons, wood charcoal none or only traces. Theref ore the fundamental composition of these two substances must be different. \T700d charcoal has t h e
